1. Field of the Invention
The invention relates to the field of inspection devices and, in particular, to a non-contacting inspection device to measure the thickness of radar absorbing materials (RAM) applied to a conducting surface.
2. Description of Related Art
A RAM coatings contains magnetic particles incorporated into a binder such as a urethane paint. The thickness of the coating must be controlled in order to obtain the proper radar absorption properties. One approach is to use a hand held thickness measuring device as disclosed in U.S. Pat. No. 5,012,248 xe2x80x9cRadar Absorption Material Thickness Measuring Devicexe2x80x9d by J. R. Munroe, et al. This invention comprises a radiating element assembly for transmitting RF energy to and recovering reflected RF energy from the coating. A visual display is provided to indicate the thickness of the coating. A portable power supply is coupled to the detector assembly making it portable. This device is highly suitable for use in checking repairs made in the field. While this device works well, it requires contact with the surface.
It is desirable to automate the application of RAM coating by use of robotic spray machines. However, since coating thickness is critical, it is desirable to check the coating thickness prior to it curing. This would make the by J. R. Munroe, et al. device unusable because of the damage to the coating that would occur upon movement of the device across the wet surface. This problem can be avoided by the use of radiating and receiving horns angled toward each other. The signal from the radiating horn is directed at the surface and the return signal is picked up by the receiving horn. However, the horns must be positioned at 12 inches from the surface. Thus the measurement is limited to relatively large areas. This prevents accurate readings of significantly curved surfaces. Furthermore, it can not be used in confined areas such as the engine inlet ducts on aircraft.
Conventional inspection techniques such as those, which use ultrasonic techniques, are unsuitable, for radar absorption is not measured, because ultrasound does not propagate well in loaded urethane or silicon based materials. Thus it is possible that the thickness may be correct, but the area may not properly loaded with magnetic materials.
Thus, it is a primary object of the invention to provide a thickness and radar performance inspection device for inspecting RAM coatings.
It is another primary object of the invention to provide a non-contacting thickness inspection device for inspecting RAM coatings
It is a further object of the invention to provide a thickness inspection device for inspecting RAM coatings that have been applied to curved surfaces.
It is a still further object of the invention to provide a thickness inspection device for inspecting RAM coatings that inspected surfaces located in confined areas.
The invention is device for inspecting an assembly that including a surface coating containing radar-absorbing materials on a conductive surface or substrate. In detail, the device includes a first circuit for transmitting an electromagnetic signal to the assembly. The first circuit includes a radio frequency (RF) source of electromagnetic radiation coupled to a waveguide made of a conductive material coupled in series to a second wave guide made of a dielectric material with their longitudinal axis aligned. A second circuit is provided for receiving the portion of the electromagnetic radiation transmitted by the first circuit reflected from the assembly. The second circuit includes a third waveguide made of a conductive material coupled in series to a fourth waveguide made of a dielectric material with their longitudinal axis aligned. The second circuit further includes a RF power detector coupled to the third waveguide. Thus an electromagnetic signal is transmitted from the first waveguide to the second waveguide on to the assembly and the portion of the electromagnetic signal reflected off the assembly is received by said fourth waveguide and transmitted to said third waveguide and to the RF power detector. The longitudinal axes of the first and second waveguides are at an acute angle to the longitudinal axis of the third and fourth waveguides. This angle is preferably 10 degrees.
The second and fourth waveguides are solid and made of a dielectric material such as a Polytetrafluoroethylene It is important to provide an impedance match between the first and second waveguides and the third and fourth waveguides, and the first and fourth waveguides to free space. This is accomplished by having the center portion of the second and fourth waveguides fit within the first and third waveguides. A portion of the second and third waveguides extend into the first and third waveguides are tapered along their top and bottom surfaces to a relatively shape edge at the end there of. A portion of the waveguides on the ends extending out of the first and third wave guides are tapered along their sides to a relatively shape edge.
The output from the RF power detector is fed to a programmable gain amplifier and thereafter to a signal digitizer. The programmable RF source and RF power detector, as well as the amplifier and signal digitizer are typically controlled by ar-microprocessor. The second and fourth waveguides maintain about 0.75 inch away from the surface of the assembly being inspected. Thus the device is typically mounted on a robotic arm, such that the assembly is automatically inspected, in a manner similar to the robotic spray machines used to apply the coating. Thus the inspection process is no different from other automated inspection systems. However, this device allows the coating to be inspected prior to its curing, .while still in a wet condition. Thus any issue associated with the material and the application process can be corrected prior to the coating curing.
The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which 1B the presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention.